Many types of measurement equipment use a column of a liquid in which the height of either the liquid or an object in the liquid must be determined. For example, one of the analytical pieces of equipment used in plastics research is a density column. This is used to very accurately measure the specific gravity of small samples of plastic. The density column consists of a graduated glass tube approximately 4 feet long and 11/2 inches in diameter. The tube is filled with the appropriate mixture of clear fluids to give a desired density range. As you go lower in the column, the density of the fluid mixture increases. The column is calibrated by using several precision density beads. These beads are pre-calibrated at specific intervals. The beads are dropped into the column and will float at various heights in the column relative to the density of the fluid. The heights are measured by reading a graduated scale on the front of the column. From these readings, a height vs. density profile is generated for the column.
To measure the density of plastic samples, they are placed into the calibrated column. Their final (floating) height is recorded. By going back to the density column profile, the density of the plastic sample can be determined. This process requires a great amount of data manipulation. This is both time consuming and can lead to errors by misreading the fine graduated scale. Therefore, it would be desirable to have an apparatus which would automatically and accurately read the column and provide an electrical signal indicating the height being read. This information would then be delivered to a computer which could automatically profile the column in the calibration mode, and give instant density readings in reading mode.